Fatigue of Automotive High Strength Steel Sheets and Their Welded Joints

Fatigue properties in various types of high strength steel sheets for automotive use and their spot and arc welded joints were investigated. In steel sheets and strain-aged sheets, fatigue limit stresses and notch sensitivities depend on not only the tensile strength but also the yield strength. Two methods for improving fatigue strength of spot welded high strength steel joints were found. One is to generate moderate expulsion during welding with a current just above the upper lobe curve and another is to conduct a controlled tempering treatment during the welding schedule. By these methods, fatigue strengths at 10⁷ cycles of spot welded high strength steel joints increase by about twice as compared with those by the conventional welding method.